A switch in aminergic modulation of locomotor CPG output during amphibian metamorphosis

In the South African clawed frog, Xenopus laevis, a complete functional switch in the mode of locomotion occurs during development from axial, undulatory, tail-based swimming in post-hatching tadpoles to limb-based kick propulsion in the adult froglet. At key stages during the metamorphosis from tadpole to frog both locomotor systems are present, co-functional and subject to modulation by the two ubiquitous biogenic amines, serotonin (5-HT) and noradrenaline (NA), arising from the brainstem. Here we review evidence on the roles of 5-HT and NA in the early maturation and dynamic modulation of spinal locomotor circuitry in the postembryonic tadpole and describe the way in which the modulatory effects of the two amines, which are always in opposition, subsequently switch during the metamorphic period of development. We speculate on the underlying cellular, synaptic and network mechanisms that might be responsible for this change in role.